1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring a polishing surface of a polishing pad during conditioning of the polishing pad.
2. Description of the Related Art
A polishing apparatus, as typified by CMP apparatus, is designed to polish a surface of a substrate by providing relative movement between a polishing pad and the surface of the substrate while supplying a polishing liquid onto the polishing pad attached to a polishing table. In order to maintain polishing performance of the polishing pad, it is necessary to condition (or dress) a polishing surface of the polishing pad regularly by a dresser.
The dresser has a dressing surface to which diamond particles are fixed in its entirety. This dresser has a dressing disk which is removable, and a lower surface of the dressing disk provides the dressing surface. The dresser is configured to rotate about its own axis and press the polishing surface of the polishing pad, while moving on the polishing surface. The rotating dresser scrapes away the polishing surface of the polishing pad slightly to thereby restore the polishing surface.
An amount (i.e., a thickness) of the polishing pad removed by the dresser per unit time is called a cutting rate. It is preferable that the cutting rate be uniform over the polishing surface of the polishing pad in its entirety. In order to obtain an ideal polishing surface, it is necessary to perform recipe tuning of pad conditioning. In this recipe tuning, rotating speed and moving speed of the dresser, load of the dresser on the polishing pad, and other conditions are adjusted.
Whether or not the pad conditioning is performed properly is evaluated based on whether or not a uniform cutting rate is achieved over the polishing surface in its entirety. In the recipe tuning, the polishing pad is actually conditioned by the dresser for several hours and a profile of the polishing pad (i.e., a cross-sectional shape of the polishing surface) is obtained. The cutting rate can be calculated from the profile obtained, an initial profile, and a conditioning time.
The profile of the polishing pad is obtained by removing the polishing pad from the polishing table and measuring thickness of the polishing pad at multiple measuring points. However, these procedures are repeated until a uniform cutting rate is obtained. Therefore, a lot of polishing pads are consumed in the recipe tuning. As a size of the substrate becomes larger, a size of the polishing pad also becomes larger. As a result, a unit cost of the polishing pad also becomes high. That is, the recipe tuning of the pad conditioning requires not only a lot of time but also a lot of cost.
The purpose of the pad conditioning is to restore the polishing surface of the polishing pad and to form a flat polishing surface. However, during conditioning of the polishing pad, the dresser may be caught by (i.e., stumble over) the polishing surface of the polishing pad, scraping away the polishing pad greatly in some parts of the polishing pad. The polishing pad with no flat polishing surface makes it difficult to planarize the surface of the substrate in its polishing process and would result in lowered yield of products.
In order to prevent the decrease in the yield of the products, it is necessary to know the profile of the polishing pad. However, obtaining the profile of the polishing pad entails the aforementioned procedures that take a lot of time and cost.